The present invention generally relates to a method of and an arrangement for improving the forming of a generally uniform paper web at an open wire or forming section of a paper-making machine, particularly where a fiber/water mixture supported by a forming wire at the forming section is advanced at high speeds which are on the order of 1000 m/min or higher.
As known in the art, the headbox of a paper machine spreads a dilute, 0.2 to 1.2% fiber/water mixture to form a layer as homogeneous as possible on a forming wire, or in a throat defined between two forming wires. Couching of the fibers to form a uniform paper web takes place in the wire or forming section, the water being drained and removed through the fiber array.
Between the outer free surface of the fiber/ water mixture moving along with the wire and the air which is stationary above the wire, there is a differential velocity equal to the speed of the wire. This differential velocity may cause undulations in the fiber/water mixture, the same phenomenon as waves on a water surface in windy weather.
Such undulations in the fiber/water mixture on the wire are disadvantageous, because they usually result in non-uniform formation of the paper web, which causes uneven letterpress printing. The undulation effect usually becomes particularly objectionable at a web speed about 1000 m/min, and it increases with increasing speed. In some instances, this undesirable undulation effect may, in fact, become a limiting factor restricting the paper machine speed.
Reference is made to the U.S. Pat. Nos. 1,563,095 and 2,716,927, and to the German Reichspatent No. 270,227, in which certain arrangements are disclosed with which a planar wire is covered and/or in which air jets are blown on top of the open planar wire. However, the mode of operation, the end use of those arrangements, as well as the design, differ considerably from the invention disclosed and claimed here.
In addition, it may be noted that at the time when the above-cited prior art patents were granted, the speeds of paper machine wires were so low that the undulation, the detrimental effects of which the present invention aims to eliminate, did not even occur, at least to no objectionable extent. Also, the quality requirements imposed on paper at that time were rather less exacting as a consequence of the letterpresses then in use.